In Lake Zu$ rich, populations of the cyanobacterium Planktothrix rubescens develop in the metalimnion during the summer and become gradually entrained in the deepening surface mixed layer during the autumn. It had previously been demonstrated that the daily integrals of photosynthetic production accounted for the growth observed in the metalimnion and greatly exceeded the smaller increase during the autumn. We have now determined the relationship between growth rate (µ) and irradiance (I ) in cultures of P. rubescens strain Pla 9316 maintained at 20mC on a 12 : 12 h light : dark cycle : the highest net growth rate averaged over the 24 h (µ h ) was 0.123 d −" , the dark rate (µ D ) was k0.020 d −" , the gross rate (φ m l µ h kµ D ) was 0.144 d −" , the affinity coefficient (α) was 0.0273 (d µmol m −# s −" ) −" and the compensation point (I C ) was 1.76 µmol m −# s −" . Using the corresponding coefficients calculated for the light period (µ Lh l 0.267 d −" , φ Lm l 0.287 d −" and α L l 0.0547 (d µmol m −# s −" ) −" ), instantaneous growth rates could be calculated from the irradiance. Comparison with growth rates at 10mC indicated a Q "! of 1.48. These coefficients were used in a modification of the Smith equation to calculate potential growth rates of Planktothrix from the irradiance and temperature at each time and depth in Lake Zu$ rich. Data on irradiance, vertical light attenuation and temperature were used to calculate the daily integrals of biomass increase over a period of 136 d. These growth integrals gave a closer correspondence to the observed population increase than the photosynthetic integrals calculated previously from measurements made with lakewater samples dominated by Planktothrix. Photosynthetic measurements made with the Planktothrix culture indicated a maximum rate of carbon increase (0.467 d −" ) that exceeds the maximum growth rate, which suggests that other factors limit growth over long periods.

The mean critical collapse pressure (p c ) of gas vesicles in 81 strains of the cyanobacterium Planktothrix rubescens from Lake Zu $ rich, Switzerland, was bimodally distributed between a minimum of 086 MPa and a maximum of 117 MPa. Measurements were made of the cylinder diameter (d) of gas vesicles isolated from seven of the strains. The mean diameter, which varied from 48 to 61 nm, was inversely related to p c , in keeping with the theory of strength of thin-walled rigid cylinders. These measurements extended the range of p c -width relationship of gas vesicles, which can be described by the expression p c l 461(d/nm) N1n53 MPa. p c was correlated with gas vesicle genotype (see the accompanying paper by S. J. Beard, B. A. Handley, P. K. Hayes & A. E. Walsby, Microbiology 145, 2757-2768) : of the 81 strains investigated, all those with the gas vesicle genotype GV2 produced gas vesicles with a mean p c of less than 10 MPa, whereas those of GV3 had a mean p c of greater than 10 MPa. It is suggested that gas vesicles of the GV3 strains, which are narrower and stronger than any previously recorded in freshwater cyanobacteria, have evolved to withstand the high hydrostatic pressures during deep winter mixing in Lake Zu $ rich.